The present invention relates to an outboard motor used to propel a boat. More specifically it relates to a four-stroke engine mounted within an outboard motor.
In general, outboard motors have an engine mounted on a plate-like engine holder. The engine contains a vertically mounted crankshaft which is rotatably connected to the lower end of the crankshaft. The crankshaft extends downward and drives a screw propeller. Four-stroke engines are more prevalent than two-stroke engines in today's outboard motors due to environmental considerations.
In a four-stroke engine, a camshaft drive mechanism transmits the rotation of the crankshaft to a camshaft located above the engine. Camshaft drive mechanisms generally use a belt drive system. A drive pulley is attached to the upper end of the crankshaft which projects from the upper surface of the engine. A driven pulley is connected to the upper end of the camshaft which projects from the upper surface of the engine. A toothed belt is wrapped around the drive pulley and driven pulley.
The engine is lubricated with an oil pump. The oil pump is mounted at the lower surface of the engine. The oil pump is driven by either the crankshaft or camshaft. The oil is drawn up from the oil pan and distributed within the engine providing lubrication. In order to prevent oil leaks, an oil seal housing containing an oil seal is located along the lower surface of the engine where the crankshaft projects downward.
In conventional outboard motors, however, substantial space must be provided above the engine because the camshaft drive mechanism is located along the upper surface of the engine. This makes it difficult to fit anything else above the engine. Additionally, in such a configuration, the length of crankshaft that must be exposed is relatively long in order to accommodate the drive pulley and fly wheel. This inevitably causes rotational vibration.
On the other hand, if the camshaft drive mechanism is located on the lower part of the engine, the height of the engine increases and this raises the center of gravity of the outboard motor. Also, it is necessary to provide a dedicated casing member to accommodate the camshaft drive mechanism along the bottom surface of the engine. This increases the cost and weight of the outboard motor. It is also more difficult to design and layout the outboard motor engine because the oil pump and the oil seal housing must be adjacent to the camshaft drive mechanism.